The invention relates generally to a method of treating angiogenesis-implicated disorders, and more particularly to a method of treating angiogenesis-implicated disorders with an endogenous aryl hydrocarbon receptor ligand or one of its analogs.
Aryl Hydrocarbon Receptor. The aryl hydrocarbon receptor (AhR) is a ligand-inducible transcription factor that mediates a broad spectrum of physiological processes upon binding to its ligand. AhR was proposed and evidenced in the 1970's (Poland et al., 1976); whereas, the coding sequence for the receptor was cloned in the 1990's, revealing that the receptor is a member of an emerging basic Helix-Loop-Helix/Per-Arnt-Sim (bHLH/PAS) transcription factor super family (Burbach et al., 1992).
Upon binding to its ligand, a liganded AhR translocates from a cell's cytoplasm to its nucleus. Inside the nucleus, the liganded AhR forms a heterodimer with Ah receptor nuclear translocator (Arnt). The heterodimer then binds to a regulatory element, Ah response element (AhRE), within target genes either to enhance or to attenuate transcription of these genes. Responses mediated by AhR include expression of P450 family genes, cell proliferation or differentiation, apoptosis, immune suppression, vitamin A depletion, inhibition of adipose differentiation, waste syndrome, vascular development and remodeling, tumorigenicity or anti-tumorigenicity, and estrogenicity or anti-estrogenicity (Schmidt & Bradfield, 1996; Alexander et al., 1998; Whitlock, 1999; Poellinger, 2000; Elizondo et al. 2000; Safe, 2001; Vorderstrasse et al., 2001; Nilsson & Hakansson, 2002; Safe & Wormke, 2003; Walisser et al., 2004; Puga et al., 2005). Additionally, AhR-deficient mice show potential physiological function of AhR in liver, heart, ovary and immune system development (Femandez-Salguero et al., 1995; Schmidt et al., 1996; Mimura et al., 1997; Benedict et al., 2000).
A Recently Identified Physiological Ligand for AhR. To date, the AhR system has been studied with artificial ligands such as polycyclic aromatic hydrocarbons including 3-methylcolanthrene (3-MC), benzo[α]pyrene (BP) and halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). While studies with these artificial ligands advance our understanding of the Ah receptor system, complete revelation of physiological roles the AhR plays and potential therapeutic benefits this system may provide necessitate the identification of its physiological ligand.
2-(1′H-indole-3′-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE) is a physiological ligand for the AhR that has been purified from procine lung and structurally identified (Song et al., 2002; U.S. Pat. No. 6,916,834). Not only has ITE been isolated from animals, but also it has been confirmed by chemical synthesis (Grzywacz et al., 2003).
Hypoxia-Inducible Factors and Arnt-Mediated Gene Expression. Hypoxia-inducible factor-1α (HIF-1α) and hypoxia-inducible factor-2α (HIF-2α) are also members of the bHLH/PAS transcription factor super family. Like AhR, HIF-1α and HIF-2α require Arnt to form a heterodimer to regulate transcription of their target genes. Under hypoxic conditions, both HIF-1α and HIF-2α are stabilized and their capability for transactivation are increased. Once activated, HIF-1α heterodimers and HIF-2α heterodimers affect transcription of over sixty genes involved in increasing oxygen delivery and activating alternative metabolic pathways (Tian et al., 1997; Giatromanolaki & Harris, 2001; Bracken et al., 2003; Quintero et al., 2004). One mechanism of increasing oxygen delivery through HIF-1α- and HIF-2α-mediated transcription is angiogenesis.
Angiogenesis. Angiogenesis is the generation of new blood vessels from existing ones and is a complex biological process involving a delicate balance of pro-angiogenic factors and anti-angiogenic factors. Pro-angiogenic factors include, but are not limited to, vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), placental growth factor (PIGF), epidermal growth factor (EGF), angiopoietins, angiogenin and angiotropin. Whereas, anti-angiogenic factors include, but are not limited to, pigment epithelium-derived factor (PEGF), angiostatin, endostatin and thrombospondin.
Under physiological conditions, such as in wound healing, in female reproduction and in cancer growth and metastasis, the pro-angiogenic factors dominate and lead to angiogenesis. The initial steps of angiogenesis include vasodilation and enhanced permeability and destabilization of a blood vessel wall. Endothelial cells, also present in blood vessels, then proliferate, migrate and form tubes. The tubes are stabilized by pericytes and vascular smooth muscle cells (Distler et al., 2003; Carmeliet, 2004).
AhR and Angiogenesis. Since AhR, HIF-1α and HIF-2α share a common dimerization partner, Arnt, and are members of the same transcription factor super family, an extensive cross-talk between them is expected. In relation to angiogenesis, AhR may compete for Amt binding with HIF-1α and HIF-2α to modulate angiogenesis (Gradin et al., 1996; Chan et al., 1999; Giatromanolaki & Harris, 2001). However, AhR interacts with other systems, such as transforming growth factor α and β (TGFα and TGFβ, respectively), epidermal growth factor receptor (EGFR), growth hormone receptor (GHR) and estrogen receptor (ER) (Hudson et al., 1986; Choi et al., 1991; Lin et al., 1991; Bryant et al., 1997; Enan et al., 1998; Zaher et al., 1998; Carlson & Perdew, 2002; Davis et al., 2003; Safe & Wormke, 2003; Nukaya et al., 2004). Therefore, AhR may modulate angiogenesis through other unknown pathways.
AhR and Vascular Development and Remodeling. Studies with TCDD, an artificial ligand of AhR, showed reduced growth of common cardinal veins (CCV) in zebrafish during a period of forty-four to sixty-two hours post fertilization (Bello et al., 2004). TCDD also blocked the regression of CCV between eighty and ninety-six hours post fertilization (Ld.). Likewise, in chick embryos, TCDD inhibited VEGF-directed vasculogenesis using coronary endothelial tube formation and outgrowth as endpoints (Ivnitski-Steele & Walker, 2003). Furthermore, AhR-deficient mice have a patent ductus venosus in their livers (Lahvis et al., 2000). Even mice expressing low levels of AhR (hypomorphs) had phenotypes similar to AhR-deficient mice (Walisser et al., 2004).
Angiogenesis-Implicated Disorders. Under physiological conditions, regulated angiogenesis occurs in wound healing, female reproductive cycles and embryonic development. Conversely, in pathophysiological conditions, unregulated angiogenesis occurs in disorders such as purpura, angioma, pallor and bone loss. Additionally, unregulated angiogenesis occurs in disorders such as growth and metastasis of cancers, eye diseases including blinding retinopathies, and chronic diseases such as psoriasis and rheumatoid arthritis.
Angiogenesis is also involved in many other disorders. For one, anti-angiogenic factors reduced adipose tissue in obese mice (Li et al., 2002; Rupnick et al., 2002). Additionally, bacterial and viral pathogens induced angiogenic genes to advance pathological processes (Meyer et al., 1999; Harada et al., 2000). Furthermore, angiogenesis occurs in atherosclerosis, restenosis, transplant arteriopathy, warts, allergic dermatitis, keloids, peritoneal adhesions, synovitis, osteomyelitis, asthma, nasal polyps, choriodal and intraocular disorders, acquired immune deficiency, endometriosis, uterine bleeding and ovarian cysts (Carmeliet, 2004).
Therapeutic Inhibition of Angiogenesis. Endothelial cells (EC) are an attractive target for anti-angiogenesis therapy in angiogenesis-implicated disorders. For one, EC are involved in the early stages of angiogenesis. Additionally, EC are in direct contact with the blood and are therefore easily accessible. Furthermore, EC are genetically stable and are a homogenous diploid cell. As such, EC are less likely to develop acquired drug resistance to anti-angiogenic therapy (Folkman, 1995; Boehm et al., 1997).
Many anti-angiogenic agents are known. These agents are generally proteins, peptides or small molecules. Examples of these agents include, but are not limited to, anti-VEGF antibodies, anti-VEGFR antibodies, inhibitors of receptor tyrosine kinases such as VEGFR1, VEGFR2, PDGFR, bFGFR and EGFR, angiostatin, endostatin, fumagilin or its derivatives, and integrin inhibitors (Eskens, 2004). Despite the existence of these anti-angiogenic agents, there is a continued demand for new anti-angiogenic agents that are safe and effective.